Immature T lymphocytes differentiating in the thymus make gene products which are found in no other normal tissue. These include a DNA polymerase, terminal deoxynucleotidyl transferase (TdT) and, in mice, the glycoprotein TL. Besides appearing transiently in normal lymphopoiesis, TdT and TL are produced by many leukemias, sometimes giving phenotypes with no normal analogue. We wish to study the molecular mechanisms that regulate expression of these products in normal development and the ways these are altered in malignant cells. Therefore, we propose to develop recombinant plasmids that will serve as hybridization probes for the RNA species that encode TL and TdT. Genomic DNA clones that include a TL structural gene have been made available to us. To use any of these as a probe to measure TL mRNA expression, it is necessary to isolate TL-specific sequences from the majority of the coding sequence which cross-hybridizes extensively with other class I genes. We are subcloning fragments derived from the TL gene and screening them for reaction with RNA species unique to TL+ thymus tissue. To obtain a probe for TdT RNA, we are attempting to use a prokaryotic expression vector (lambda gt11) and an immunological screening technique to detect TdT sequences expressed by the recombinants. The cloned probes will be used on Northern blots to identify TL and TdT RNAs from tumor cells of different phenotypes and from separated subpopulations of normal thymocytes.